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Midway through the film, there is a famous continuous shot in which the camera backs away from the door of Rusk's upper-floor apartment and descends the staircase, seemingly without a cut, to the ground level, out the building's front door, and then to the opposite side of the street. The interiors were shot with an overhead track in a studio, and there is an imperceptible cut as a man passes by the front door, carrying a sack of potatoes. This is subtly blended into a new shot of the camera pulling away from the building exterior that was actually used on location.

My first reaction was it was done with Steadicam, except Frenzy came out in 1972 and Steadicam was first used in 1976.

I found a good description of how the shot was done here: "Deconstructing the Tracking Shot". A camera jib was used for the interior shot, and a dolly ws used for the exterior. There is actually an edit in the shot, hidden by the man walking in front of the camera just as the camera exits the building. It is there the shot switches from the jib to the dolly.